MOSCOW, Oct 23 (Reuters) - Russia on Wednesday dropped piracy charges against 30 people involved in a Greenpeace protest against Arctic oil drilling, replacing them with lesser offences and cutting the maximum jail sentence they face to seven years from 15 years.

The charges against activists who protested at a Gazprom oil platform off Russia's northern coast last month have been changed from piracy to hooliganism, the federal Investigative Committee said in a statement.

Greenpeace said the new charges were still "wildly disproportionate" and promised to contest them.

All 30 people who were aboard the Greenpeace ship Arctic Sunrise during the Sept. 18 protest, in which activists tried to scale the Prirazlomnaya platform, are being held in detention in the northern Murmansk region until at least late November.

The Investigative Committee said it had begun the procedure of pressing the new charges, which carry a maximum sentence of seven years in prison. The piracy charges were punishable by 10 to 15 years.

Greenpeace said the hooliganism charge "represents nothing less than an assault on the very principle of peaceful protest".

"This is still a wildly disproportionate charge that carries up to seven years in jail," Vladimir Chuprov of Greenpeace Russia said in a statement.

"We will contest the trumped up charge of hooliganism as strongly as we contested the piracy allegations. They are both fantasy charges that bear no relation to reality," he said.

"The (activists) are no more hooligans than they were pirates," he said, adding that they had "protested peacefully against Gazprom's dangerous oil drilling and should be free."

Russian President Vladimir Putin has said the activists were clearly not pirates but that they violated international law.

MORE CHARGES POSSIBLE

The Investigative Committee dismissed Greenpeace's claim that the protest was peaceful, saying "anyone who illegally and premeditatedly seizes ... a stationary platform is committing a crime, no matter what their motive."

The committee said the investigation was continuing and reiterated an earlier statement that it could still bring additional severe charges against some of the activists, including the use of force against representatives of the state.

Courts in the Russian city of Murmansk have denied bail to the 30 people detained - 28 activists, including the crew of the Arctic Sunrise, and two freelance journalists who were documenting the protests.

Greenpeace has said the arrests and charges are meant to frighten off campaigners protesting against drilling in the Arctic, a region Putin describes as crucial to Russia's economic future and its security.

Moscow says the environmental protesters violated a security zone around Prirazlomnaya, which is Russia's first offshore oil platform in the Arctic and is scheduled to begin production by the end of the year after delays.